"If something happens, God forbid, (then) Chernobyl which we all know about very well would simply seem like a children's fairy tale," he told reporters at an industrial fair in Germany.

Speaking at a joint press conference with Chancellor Angela Merkel, Putin welcomed a US decision to delay the test of an international ballistic missile to avoid stoking tensions with North Korea.

"I think the United States took a very important step in delaying the test of a ballistic missile," he said.

"We should thank the United States for this important step," he added. "I hope it will be noticed by our partners including North Korea and that everyone will calm down and work together to seek a solution to the various problems."

With an eye to China, Merkel said world powers with influence in the region needed to work together to smooth tensions.

"We agreed that the international community must work to have a calming effect on the situation and toward North Korea stopping its provocations," she said.

North Korea's bellicose rhetoric has reached fever pitch in recent weeks, with near-daily threats of attacks on US military bases and South Korea in response to ongoing South Korea-US military exercises.

The United States, which has met the North's threats with some military muscle-flexing of its own, offered a calibrated concession Saturday by delaying a planned inter-continental ballistic missile test.

Washington had said it feared the Minuteman 3 test could be misconstrued as an attempt to exacerbate the crisis.

China, Pyongyang's sole major ally for decades and its biggest trading partner, said Sunday it had told North Korea it would not tolerate "troublemaking" on its doorstep.

IAEA chief calls N.Korea actions 'troubling'
Washington (AFP) April 8, 2013 - The UN atomic watchdog chief on Monday called North Korea's drive to restart a nuclear site "troubling" and said his team could not detect whether the regime planned a new nuclear test due to a lack of access.

Yukiya Amano, director general of the International Atomic Energy Agency, said the watchdog had observed Pyongyang's efforts to rebuild parts of its Yongbyon nuclear site amid a showdown with South Korea and the United States.

"That is very troubling because they are against United Nations Security Council resolutions," Amano told reporters at a conference on nuclear policy held by the Carnegie Endowment for International Peace in Washington.

Amano said the IAEA had little knowledge of North Korean activities and relied largely on satellites since Pyongyang kicked out the agency's staff in 2009.

"Our knowledge on the activities of North Korea is rather limited. I cannot speculate when and if North Korea will conduct another nuclear test," Amano said.

South Korea said earlier that North Korea appeared to be preparing its fourth nuclear test, as well as a missile launch. But South Korean officials later backtracked, saying activities at the North's Punggye-ri test site were routine.

Tensions have soared in recent weeks over North Korea, which has responded furiously to United Nations condemnation of its nuclear test in February and its launch of a long-range rocket that put a small satellite into orbit.

North Korea said last week that it would restart all facilities at Yongbyon, where it had knocked down a cooling tower in 2008 to show it was serious about US-supported talks on denuclearization.